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Publisher Summary This chapter explains a new prostaglandin pathway distinct from that which forms PGE2 and PGF2α, now referred to as the “prostacyclin pathway.” The formal naming of products in the prostaglandin family is based on the prostanoic acid skeleton. The prostacyclins are no exception. Thus, the bicyclic product now known as “prostacyclin” was referred to initially as “6(9)oxy-11,15-dihydroxy prosta-5,13-dienoic acid” and subsequently when the stereochemical assignment of the double bonds was established as 9-deoxy-6,9α-epoxy-11,15-dihydroxyprosta-5Z, 13E-dienoic acid. Prostacyclin is generally referred to the product derived from arachidonic acid; consistent with the alphabetic naming of prostaglandins, this product has been assigned the abbreviated form PGI2. While prostaglandins are derived from all cis essential fatty acids having 3, 4 and 5 double bonds, a cis double bond at carbon 5 of the fatty acid is essential for prostacyclin synthesis from the precursor endoperoxide. A factor with biological properties similar to PGI2 is spontaneously released by the lungs. This release can be enhanced by Angiotensin I and II but not by norepinephrine or vasopressin.